Tips for increasing oscilloscope vertical resolution - Part I

Resolution is the smallest quantization level determined by the oscilloscope. An 8-bit ADC can encode an analog input to one in 256 different levels, since 28=256. The ADC operates on the scope’s full scale vertical value. Thus, the Q-level steps are associated with the full-scale vertical scope setting. If the user adjusts the vertical setting to 100mV per division, full screen equals 800 mV (8 divisions * 100 mV/div) and Q-level resolution is equal to 3.125 mV/level (800mV divided by 256 levels).

Scaling the waveform to take the whole display of the scope enables you to use more of the scope’s analog-to-digital (ADC) converter. If a signal is scaled to take up only ½ of the vertical display, you’ve just decreased the number of ADC bits being used from 8 to 7. Scale the waveform to ¼ of the vertical display and you’ve reduced the number of ADC bits used from 8 to 6. Scale the waveform to take close to consume full vertical scale and now you are using all 8 bits of the oscilloscope’s ADC. Use the most sensitive vertical scaling setting while keeping the waveform on the display.

31 mV resolution

16.5 mV resolution

Figure 2: Many scopes allow multiple grids such as Agilent’s Infiniium where users can have up to four simultaneous grids. This allows for easier viewing while scaling the waveform to use the entire ADC dynamic range

Many scope vendors allow users to populate the scope with multiple grids. This is done simply to allow users who want to see individual waveforms instead of overlaying the waveforms. One or more waveforms can be placed and scaled in each graticule making the scope display easier to view. Each waveform can be scaled for full-scale vertical value within a grid as shown in figure 2.

Do more scope ADC bits enable you to see small signals? Theoretically, yes. In practice, scopes with 12-bit ADCs have noise levels that are far able the smallest quantization levels. Hence, not all 4096 levels can be used as the least significant digits are just quantizing noise. 8-bit ADCs using high-resolution mode achieve the same noise levels as scopes with 12-bit ADCs. This is because the quantization noise is overshadowed by the front-end noise of the scope.